1. Field of the Invention
The present invention relates to a spindle stock unit suitable for use in an ultra-precision surface grinding machine.
2. Description of the Prior Art
In a conventional spindle stock unit, a spindle is rotatably supported by a pair of axially spaced radial fluid bearings in a spindle head and positioned in place by a pair of axially spaced thrust fluid bearings in the spindle head. The spindle is, in general, made of quenched ferrous metal and finished by grinding in a predetermined configuration. During rotation of the spindle, the bearing surfaces of the spindle are heated by fluid friction to cause thermal expansion of the spindle, resulting in axial displacement of the spindle at its opposite ends. In use of such a spindle stock unit in an ultra-precision surface grinding machine, a grinding wheel is fixedly mounted on one end of the spindle in such a manner that the grinding surface of the wheel is arranged perpendicularly to the axis of the spindle to grind the surface of a workpiece on a table of the grinding machine. In such arrangement of the grinding wheel, it is not possible to ensure precise feeding of the grinding wheel toward the workpiece due to axial displacement of the spindle caused by thermal expansion.